Density functional theory assessment of transannular N⋯Y interactions in some medium-sized heterocycles

Transannular interactions involving a nitrogen and groups Y in ten-membered rings based on the alkaloid protopine were studied by DFT methodology to investigate the potential mimicking of shapes and electrostatic potentials and thus biological activities of fused berberine and related analogues. The methodology was confirmed for protopine (Y[dbnd]CO) and the transannular N⋯CO interaction rationalised as an n → π* interaction. With Y[dbnd]CS, the same interaction was observed, as well as structures with N–C single bonds. With Y[dbnd]BOH, structures with dative N–B single bonds were found. In all three cases, the structures with transannular interactions were more energetically favourable than “wide” conformations. To quantify the interactions, topological density-based descriptors, natural bond orbital charges and overlap energies and electrostatic potential-derived (ESP) charges were calculated. Visualisation of electron densities and ESPs mapped onto the densities was explored. The scope of the transannular N⋯Y interaction was examined with an additional five Y groups.